1. Field of the Invention
The present invention relates generally to a PTC thermistor, and more particularly, to a PTC thermistor contained in a DC motor or the like and used for the purpose of, for example, the prevention of an overcurrent.
2. Description of the Related Art
FIG. 7 is a perspective view showing a conventional PTC thermistor in which electrode layers 1 are provided on both major surfaces of a ceramic body 2 having a positive temperature coefficient resistivity. A reinforcing plate 3 made of a metal is provided on the electrode layer 1 through a conductive adhesive layer 4 composed of conductive adhesives. A material, such as copper, brass or the like, having good heat dissipation properties and conductive properties is used as the reinforcing plate 3. Such a reinforcing plate 3 reinforces the ceramic body 2 and increases the volume of the entire PTC thermistor so as to extend operating time.
The conventional PTC thermistor self-heats so that its resistance value is increased by its positive temperature coefficient of resistivity when, due to an abnormality, an overcurrent exceeding a predetermined current value flows through the device, thereby operating to keep the flowing current below the predetermined current value. .Such a PTC thermistor is incorporated in a DC motor for the purpose of, for example, prevention of an overcurrent in the DC motor. FIG. 5 is a perspective view showing such a DC motor. FIG. 6 is a cross sectional view taken along a line A--A shown in FIG. 5. Referring to FIGS. 5 and 6, brushes 12 for supplying power to a commutator 11 of an electrode (not shown) are supported on respective ends of conductive inner terminals 13 and 14 inside a DC motor 9. The PTC thermistor 15 is interposed between the other end of the inner terminal 14 and the inner terminal 16. The inner terminals 13, 14 and 16 are respectively mounted on a motor case 10.
The PTC thermistor shown in FIG. 7 is used, as one of its applications, for preventing an overcurrent from flowing in the DC motor as shown in FIG. 6.
In the conventional PTC thermistor, having the reinforcing plates as shown in FIG. 7, a metal plate is used as the reinforcing plate. Since the ceramic body and the associated metal plate has very different coefficients of thermal expansion, the following problems occur.
Specifically, if conductive adhesives or the like are used for joining the ceramic body and the associated reinforcing plate to each other, stress is produced due to the difference between the coefficients of thermal expansion, and the stress is applied to the ceramic body. As a result, the ceramic body is damaged.
Furthermore, as the PTC thermistor self-heats, stress based on the difference coefficients of thermal expansion of the reinforcing plate and the ceramic body; the stress is applied to the PTC thermistor. Therefore, the employment of the PTC thermistor may be a crack and could result in being formed in the PTC thermistor. Additionally, the conductive adhesive layer between the reinforcing plate and the ceramic body, for example, could also be cracked, so that the functioning of the PTC thermistor is degraded.